All suspects detained in connection with the Westminster attack have been released with “no further action”.
The Metropolitan Police said the final man in custody, a 30-year-old from Birmingham who was originally held on suspicion of preparing terrorist acts, was freed on Saturday.
Eleven others arrested in connection with the investigation have previously been released, with Scotland Yard saying “inquiries continue”.
Investigators have been searching for potential accomplices who may have helped Khalid Masood plan or carry out his attack.
Suspicions over a potential wider extremist network intensified after it emerged he used WhatsApp just minutes before ploughing his car into pedestrians on Westminster Bridge.
British mother Aysha Frade, pensioner Leslie Rhodes and American tourist Kurt Cochran died of injuries sustained during the rampage.
Masood then crashed into railings outside the Houses of Parliament and ran into an entrance where he stabbed PC Keith Palmer to death, before being shot dead by armed police.
Investigators believe Masood was inspired by Islamist terrorism and Isis has claimed responsibility for the massacre, although the extent of the group’s involvement is not yet known.
In pictures: Westminster attack
In pictures: Westminster attack
An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London
MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house.
Crowds gather in Westminster after shooting incident, which police are treating as terror attack
Police were also called to an incident on Westminster Bridge nearby
Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner
Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife
Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident
One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital
At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended
Scotland Yard’s acting deputy commissioner, Mark Rowley, said police were probing whether the father-of-three acted completely alone after being inspired by terrorist propaganda or was “encouraged, supported or directed” by others.
Deputy assistant commissioner Neil Basu acknowledged that the method of attack, following Isis propaganda instructions on using cars and knives, “echoed Isis rhetoric” but said investigators had not yet found any evidence of a direct link.
“Whilst I have found no evidence of an association with Isis or al-Qaeda, there is clearly an interest in jihad,” he added.
Questions over whether Masood may have been radicalised in part by extremist material online have sparked renewed efforts to remove such content, seeing the Home Secretary meet with technology companies this week.
Investigators continue efforts to establish the motive of Masood, a 52-year-old Muslim convert who had a lengthy criminal history and lived in areas including Sussex, Birmingham and London.
Thousands of people joined hands in a vigil for the victims exactly a week after the attack on Wednesday, while further tributes have been paid across the country.
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